Organic materials have shown promise as the active layer in organic based thin film transistors and organic field effect transistors (OFETs). Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
A suitable semiconducting material known from prior art is regioregular head-to-tail (HT) poly-(3-alkylthiophene) (P3AT), such as poly-(3-hexylthiophene) (P3HT). It shows a high charge carrier mobility, a good solubility in organic solvents, and is solution processable to fabricate large area films.
The synthesis of regioregular polythiophene is well reported in the literature by a variety of methods including Grignard metathesis [1], Rieke coupling [2], Stille coupling [3], and Suzuki coupling [4]. Further methods can be used to obtain regioregular polythiophenes only when the monomer employed is symmetrical, namely Yamamoto coupling [5] and oxidative polymerisation [6]. However, these methods currently suffer from some drawbacks, particularly when synthesising copolymers comprising at least two thiophene-based monomers.
The Grignard metathesis, Rieke coupling, Yamamoto coupling, and oxidative polymerisation methods are all only applicable to the synthesis of regioregular homopolymers based upon a single monomer unit. These synthetic approaches are therefore not applicable to the preparation of alternating copolymers. Furthermore, the Grignard metathesis so far has proven to be useful only for the synthesis of P3AT.
On the other hand, the Stille and Suzuki coupling methods are amenable to the preparation of both regioregular homopolymers and alternating copolymers, but yet each suffers from a critical obstacle.
In the case of the Stille coupling, a main obstacle is the toxicity of organotin compounds. Typically either tributylstannyl or trimethylstannyl thiophene derivatives are employed in the polymerisation. These monomers are prepared from a trialkyltin halide, in particular trimethyltin halide, which is highly toxic. The trialkyltin halide is also re-generated during the polymerisation. The toxicity issue is disadvantageous especially in the large-scale preparation of polythiophenes via this route.
For the Suzuki coupling, the current obstacles are the low molecular weight polythiophenes obtained [7] and/or the low yields [4,7]. This is due to significant deboronation of thiophene boronate esters/acids occurring during the reaction [7], which typically limits the molecular weights obtained.
The molecular weights of polythiophenes have a direct effect on their thin-film morphology and consequently on their field-effect mobility as observed for P3HT [8]. In addition, it was reported that the performance of (bulk heterojunction) solar cells depends on the molecular weights of P3HT used in the device [9]. Thus, it is critical to be able to synthesise high molecular weight polythiophenes, in order for them to be an attractive candidate for application in electronic devices, such as FETs and solar cells.
The prior art [10, 11, 12] also discloses Suzuki coupling methods that yield high molecular weight polymers (Mn>100,000 Da) in good yields, for example for 9,9-dialkylfluorene based copolymers. However, these methods do not yield polythiophenes with high molecular weights and in high yields.
Therefore, there is still a need for an improved method of preparing thiophene polymers with high regioregularity, high molecular weight, high purity and high yields in an economical, effective and environmentally beneficial way, which is especially suitable for industrial large scale production.
It is an aim of the present invention to provide an improved process for preparing thiophene polymers with these advantages, but not having the drawbacks of prior art methods mentioned above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
It was found that these aims can be achieved with a process as claimed in the present invention. This process enables coupling thiophene and selenophene compounds and is especially suitable for synthesising conjugated polymers of thiophene or selenophene with high molecular weight and high regioregularity. The polymers obtianed by this process are useful as semiconducting component especially in the fabrication of FETs and TFTs.